1. Field of the Invention
The present invention relates to a technique for removing a noise component included in an image.
2. Description of the Related Art
In conventional image reading apparatuses such as a digital copying machine and a scanner, a filter unit for removing isolated points such as a stain in an image is used for an image processing. As a method for efficiently removing a noise, for example, Japanese Patent Application Laid-Open No. 5-48893 discusses the following image signal processing method.
In the image signal processing method discussed in Japanese Patent Application Laid-Open No. 5-48893, first, difference values between an intermediate signal value of three signal consecutive values and signal values adjacent to the intermediate signal value are calculated. Next, an average value between the two signal values adjacent to the intermediate signal value are calculated. The intermediate signal value is corrected to a signal value based on the average value when each of difference absolute values is greater than a predetermined reference value and polarities of the difference values between the intermediate signal value and the signal values adjacent to the intermediate signal value are inverted from each other. An isolated point noise can be removed without influencing on a high frequency component of image signals by performing such signal processing on entire pixels.
However, in some cases, the filter processing with using a low pass filter (LPF), a scaler, or the like may cause a blur like a gradation in a range of a few pixels in the vicinity of the isolated point noise. In such cases, the conventional noise detection method in which pixel values of pixels adjacent to a target pixel are used as operands has an issue of deterioration of detection accuracy of the isolated point noise since the blurred potion is also contained in a comparison reference range in the conventional method. The conventional method also has an issue that the isolated point noise component is included in a pixel to be corrected since the blurred potion in the vicinity of the isolated point noise is included in the calculation range in the case of calculating the pixel to be corrected.